1. Field of the Invention
The present invention relates to gauge systems for indicating the level of liquid lading in a tank and, particularly, to electronic gauges of the type which can produce an alarm signal and/or an indication of tank liquid level. The invention has particular application to a sensing head for such a gauge system.
2. Description of the Prior Art
The present invention is an improvement of tank level gauge systems of the general types disclosed, e.g., in U.S. Pat. Nos. 5,124,686 and 5,585,786. Both of these prior systems are float gauges, which utilize a float which rises and falls with the liquid level along a fixed guide tube, which extends into the tank from the top thereof. The float carries a magnet for magnetic coupling to a magnet adjacent to an end of a gauge rod or tube, which is movable longitudinally within the guide tube so that the rod rises and falls with the float. U.S. Pat. No. 5,124,686 discloses a magnetic system, which has a pair of spaced magnets adjacent to the top end of the rod, which actuate a reed switch in a sensor head at the top of the guide tube. A similar pair of magnets may be disposed adjacent to the bottom end of the rod. At least one other magnet is disposed intermediate the ends of the gauge rod for indicating significant tank levels when sensed by the reed switch. U.S. Pat. No. 5,585,786 discloses an optical system, wherein the gauge rod is provided with a code pattern including a plurality of longitudinally alternating light and dark circumferential stripes. In this case the sensing head has a light source to illuminate the rod and photosensors arranged in quadrature relationship to detect the passage of the stripes to indicate rate and direction of movement of the rod. The ends of the rod have elongated dark regions to indicate these reference positions.
While these prior systems work reasonably well, they have certain disadvantages. The magnetic system provides only gross indications of a few significant liquid levels. While the optical system is capable of providing a relatively precise and continuous indication of liquid level, the optical system has proven to be unreliable in detecting the ends of the gauge rod, resulting in spurious readings.